Vibratory gyroscopes are well known in the art (for example, reference U.S. Pat. Nos. 4,019,391 and 4,644,793). Such gyroscopes are considerably more rugged than conventional spinning wheel gyroscopes, can be started much more quickly, consume less power and have no bearings (which could be susceptible to wear). A wide variety of vibrating members have been employed in the vibrational gyroscope art, ranging in shape from a tuning fork to a pair of torsionally oscillating coaxial spoked wheels. However, most existing rotary rate gyroscopes are complex devices far less suited for micromechanical scaling.
Many applications exist for extremely small rate gyroscopes that exhibit low drift and offset errors and that are cost effective and rugged. These include head position sensors to control virtual reality displays, control of miniature motors and mechanical arms, control for a ground vehicle or vibration-cancelling suspension, angular accelerometers for guidance systems serving small vehicles or pedestrians, and miniature missile proportional-navigation homing seekers.
Different applications place different requirements on gyroscope system performance and/or application-specific processing. For example, in virtual reality displays or personal guidance systems, the range of rotation rates and the maximum permissible latency in estimation of angle rate are moderate, and the drift must be very low. In contrast, in a proportional-navigation homing seeker the angle-rate range is very large, the latency must be very low and drift rates may be moderately large before system performance degrades.
Micromechanical gyroscopes which are micromachined from a single substrate are well known in the art. By way of example, an angular rate sensor made out of a single piece of quartz was studied by J. Soderkvist and discussed in an article entitled "Design of a Solid-State Gyroscopic Sensor Made of Quartz," Sensors and Actuators, A21 A23, pp. 293-296 (1990). The piezoelectric effect in quartz is used both to excite a reference vibration in the plane of a tuning fork and to sense the vibration normal to this plane due to an externally applied rotation. The amplitude of the second vibration is directly proportional to the angular velocity of the applied rotation.
Another type of micromechanical gyroscope is described by J. Bernstein in U.S. Pat. No. 5,203,208. This Bernstein gyroscope includes a mass symmetrically supported by both drive and sense axes, for detecting rotational movement about an input axis. The gyroscope includes an inertial mass supported by two pairs of flexures. Each pair of flexures is attached to diametrically opposed sides of the inertial mass and a gyroscope support frame. Additionally, each of the flexures is oriented at generally a 45.degree. angle from both the drive axis and the sense axis. In response to an applied drive signal, the inertial mass is induced to vibrate about a drive axis which is co-planar with and orthogonal to the sense axis. Both pairs of flexures participate equally during rotation of the mass.
Still another type of micromechanical gyroscope known in the art comprises a gimbaled structure which includes an inner gimbal ring having a set of flexures coupled to a mass. The inner gimbal ring serves as the sense axis. The innter gimbal ring is located within an outer gimbal ring which serves as the drive axis and is coupled to a gyroscope frame by an outer set of flexures.
Most, if not all, prior micromechanical gyroscopes (including the Bernstein gyroscope) are complex to implement and generally require a relatively large substrate surface area, e.g., on the order of hundreds of microns. Such devices are of a scale significantly larger than today's integrated circuits. Thus, a scaled vibratory rate gyroscope with dimension on the order of microns (for greater compatibility with existing integrated circuits), and which allows performance differences to be addressed, is believed particularly commercially desirable. The present invention provides such a micromechanical gyroscopic sensing device.